All aircraft potentially have flammable atmospheres in the fuel tanks and in adjacent flammable leakage zones. This is a primary safety issue in aircraft design and operation. In order to avoid flammability, polymeric hollow-fiber membrane gas separation is being used to decrease the oxygen concentration below 12% to create an effectively inert atmosphere. However, the current state-of-the-art polymeric membrane materials are bounded by Robeson's upper bound, which limits the improvements in lowering the membrane surface area and in increasing membrane efficiency. In the last fifteen years, carbon hollow fiber membranes have emerged as materials with performance that exceeds the upper bound of polymeric membrane materials. Early research showed sensitivity to oxygen and water vapor, which would deem the use for air separation impractical. However, more recent research has shown that these issues could be overcome by Teflon@ based coating, oxygen doping, or otherwise modifying the pyrolysis environment.
CMS membranes are generally produced by the pyrolysis of polymer fiber membranes under an inert atmosphere or vacuum. The ultimate properties of CMS membranes are affected by many factors, such as polymer precursor composition, pyrolysis temperature, ramp rate, and thermal soak time at the maximum pyrolysis temperature. The effect of pyrolysis atmosphere has been investigated by several researchers, and it is found that even a trace amount of oxygen in the pyrolysis atmosphere is important in defining transport properties of CMS membranes. This so-called “oxygen doping” has been used during pyrolysis as a tool to fine tune the selective pore windows (ultramicropores) of CMS membranes for natural gas separation, where O2/Argon mixed gas was used in place of a totally inert atmosphere during pyrolysis. A range of oxygen levels varying from 4-50 ppm with the balance comprising argon were used to prepare CMS membranes, and a correlation was found between the amount of oxygen introduced at the temperature of pyrolysis and ultimate separation performance of CMS membranes. The developed method relied on the fact that oxygen molecules chemisorb at ultramicropores, where the open carbon bonds are available at the edges (dangling bonds) and form stable bonds at high temperature. The hypothesis was supported by the experimental results, where the permeability of carbon dioxide decreased with an increase in selectivity up to a point when excessive doping reduced both permeability and selectivity. The applicability of the method was tested for different polymer precursors also. While trace residual elements exist in the carbon molecular sieve materials for pyrolysis temperatures below 1073 K (800° C.), the FTIR spectra are almost featureless in the absence of O2 doping. Even at 10 ppm O2 in the otherwise inert Argon gas used during pyrolysis the FTIR shows some evidence of C═O carbonyl groups. With increasing O2 doping in the Argon, the C═O group absorbance increases without any other apparent groups appearing.